Sodium and potassium specific microelectrodes will be used to determine intracellular activities of these ions in epithelial absorptive cells of the isolated small intestine of Amphiuma means. Lower than normal buffer sodium concentrations will be used to provide a range of sugar transporting ability (determined by 3-0-methyglucose accumulation) and the energy required for sugar accumulataon compared with that available in the chemical and electrochemical ionic gradients. This will provide a direct test of the sodium gradient hypothesis for active sugar transport in the intestine. The same activities will be measured in the absence of potassium and in the presence of high potassium concentrations and the inhibition of sugar accumulation examined in terms of alterations in the same chemical and electrochemical ionic gradients. The relative permeability of the mucosal membrane to sodium and potassium will be calculated by the Goldman equation and predictions tested by alteration of the diffusion potentials for these ions. The electrical coupling between the mucosal membrane and the serosal membrane will be studied at low sodium concentrations by measuring the change in mucosal and transmural potentials produced by sugar. Ouabain in high concentrations will be used to abolish the transmural potential and the temporal alteration of the individual membrane potentials examined in terms of a possible electrogenic sodium pump at that membrane. This point will be examined further by measuring the membrane and transmural electrical responses to sugar as inhibition of sodium transport progresses in the presence of low ouabain concentrations.